A PEM fuel cell module includes a fuel cell stack. The fuel cell stack contains numerous polymer electrolyte membranes, alternatively called proton exchange membranes, which conduct protons between the electrodes. A PEM fuel cell is powered by a first reactant comprising oxygen, for example air, and a second reactant comprising hydrogen, for example essentially pure hydrogen or methane. Other types of fuel cell modules are also known.
A fuel cell module may be combined with a battery to provide a hybrid electrical power supply system. For example, hybrid power systems can be used to provide a back-up power supply in case of grid failure. In another example, a hybrid power system can be used to power a vehicle. In these cases, and others, it would be desirable to be able to start a fuel cell in a short period of time. In the case of a back-up power supply, the battery is required among other things to provide enough storage to carry anticipated loads until the fuel cell is started.
Currently available fuel cell modules typically start in about 20-60 seconds. The start time is measured from the time of a start command until the module delivers power to a load or reaches a specified percentage, for example 80% or 100%, of its rated power output.
A fuel cell produces voltage according to a polarisation curve. The polarisation curve describes the fuel cell voltage as a function of the fuel cell current or the fuel cell current density. In general, as current supplied by the fuel cell increases from zero, the fuel cell voltage initially drops rapidly through an activation region, then drops nearly linearly through an ohmic region, then drops more rapidly through a mass transport region. A battery typically has a different polarisation curve and so at some times, for example when a fuel cell is starting or disconnected, a battery and fuel cell may have incompatible polarisation curves. In some cases, a battery and fuel cell are connected through a DC to DC voltage converter to help manage differences in their polarisation curves.